SSD is the trend right now in the market, and many hosting companies usually go for SSD.
Better read their constraints before investing as Unlimited also has some limits.
As others have mentioned, there's often times a catch or fine print with hosting plans that offer unlimited storage. If you're considering a hosting plan with unlimited storage (and truly need lots of storage), you should check their Terms Of Service and Fair Usage Policy for certain restrictions, such as limits regarding I/O usage, IOPS, processes, CPU usage, etc.
The hosting companies offering unlimited disk will usually have a lot of conditions attached to it. In most cases, their condition is that the space can be used only for the website. For example, you cannot use the space as a sort of offsite backup space for your home or office computer.